Like Darwin, I’ll start with an obvious example of how we have affected the evolution of species: domestication. Cows, sheep, pigs, chickens et al. resemble their wild ancestors but in many ways are very different. These animals now produce more meat, lactate more and are a lot more docile than their progenitors. This isn’t just something that we’ve done to animals but also plant: modern wheat looks very different from the grasses that hybridised to produce its ancestor and maize is hugely different from teosinte. Since the advent of agriculture, humans have selectively bred the plants and animals with the characteristics they desired the most and this has even happened with pets, leading to some ridiculous breeds.
All of the examples above are ways that humans have influenced evolution either intentionally or in ways that we’ve directly benefited from. Another common example that is often in the press, but which we don’t benefit from, is antibiotic resistance. Through our use of antibiotics to prevent and cure infections and diseases we have created a huge selection pressure on pathogens, specifically bacteria, to develop resistance to these drugs. This has resulted in antibiotic resistance. We’ve changed the environment and created a massive advantage to those individuals whose mutations convey resistance. This is evolution on super drive, as bacteria reproduce rapidly, one bacteria could produce a billion descendants in 10 hours (if conditions are right), and can transfer genes between species, further accelerating the process.
Human activity is directly impacting on the evolutionary trajectories of species that are much bigger than bacteria. Humans are a super predator, able to take large numbers of a species at a fast rate and not switching when their numbers fall. This form of predation creates a huge selective pressure on the prey species to change. Atlantic cod, the species that Brits prefer as part of our fish’n’chips used to be over 2m long. By removing large fish, through commercial fishing, we’ve created an environment were being smaller and reaching sexual maturity earlier is optimal. This human induced change in the environment has led to cod evolving to be smaller and that males are becoming sexually mature earlier. But this doesn’t just mean that our fish’n’chips will decrease in size (it’s not a Tory plot) but it changes the whole ecosystem as cod’s place in the food chain will change, now becoming prey when it once was a predator.
Predation doesn’t just impact the evolution of the species we exploit commercially but also those that we hunt for fun. This is something that I’ve seen first-hand while working in South Africa. We shot antelope on our ‘farm’ solely for food, and as such just took the first ones we found (if they weren’t pregnant). As such, our male springbok hand lovely big, symmetrical horns, because the predation pressure was in effect random. Our neighbours, who often had trophy hunters pay to come onto their property and pay to shoot males with big horns had male springbok with smaller, less impressive horns (even when of comparable size). By selectively shooting males with large horns humans had produced a change in the predation regime that made having big horns very costly and so selected for males with smaller horns. This isn’t just anecdote; trophy hunting has been shown to have reduced the size of bighorn sheep’s horns by about 25%. This isn’t just an impact on the physical appearance of males but also of the way that mate selection takes place. Prof David Coltman, author of this research, explains: "The crux here is that the horns develop to the length where they can be legally harvested several years before they achieve social dominance. So in effect they're being harvested from the population before they can reproduce and pass their genes on."
Urban environments are not devoid of nature. Many bird species are common in towns and cities throughout the world. However, living in a habitat that differs so much from the one that these species have evolved in has led to many behavioural and physiological changes. One of the obvious ones is that bird song in urban areas differs to the song of the same species in more ‘natural’ habitats. Birds sing at different times, even at night, to avoid noise interference. The actual acoustic structure of bird song has also changed in urban environments, and this is a crucial aspect of not just mate selection but territorial behaviour in many of these species. The optimal behaviour of a species in an urban area can be different to that of individuals of the same species in wilder areas, urban birds may not migrate like their ‘wilder’ brethren and this is an anthropogenically driven adaptation. An finally, urban areas may act like islands because they are invaded by a small number of individuals of a species creating founder effects but also because urban individuals do not regularly breed with their wild neighbours, effectively isolating themselves off. This has happened in the European blackbird and resulted in reduced genetic diversity of urban populations but also resulted in physical differences between urban and rural birds.
Adding to all of this is the increased movement that human activity has allowed certain species. This has meant that species can move between continents like never before, creating novel ecosystem assemblages and interactions. The presence of alien species, which are often well adapted to human disturbed environments, offers new unexplored avenues for evolution to try out. This is a lot more speculative than the previous paragraphs, but new mutualisms, host-parasite relationships and even hybrids may be created because humans have allowed animals and plants to move further and faster than ever before.
This is both fascinating and frightening at the same time. But without studying these impacts it is not possible to gauge how we can reduce/stop them, or even enter a debate into whether we need to intervene at all? Are we not just another part of the environment?